Phosphate metabolism and intracellular pH will be studied noninvasively with 31p NMR spectroscopy in isolated rat hearts during experimental cardioplegia and reperfusion. A major guideline will be to simplify the system by reducing the number of variables, permitting thereby a more clearcut interpretation of the results than in protocols resembling the performance of cardiac surgery in man. Preliminary data with isothermic (25C), nonworking hearts reveal quantitative, more clearly interpretable results which differ from those obtained in studies conducted with hypothermic (15C) cardioplegia and normothermic (37C) recovery of hearts performing work. Experiments will be performed in which Ca2+, Mg2+, temperature, and work will be varied systematically during arrest as well as during recovery. The involvement of Ca, Mg-ATPase will be investigated with the inhibitor, trifluoperazine. The prevention of adenine nucleotide degradation with the adenosine deaminase inhibitor, 2'-deoxycoformycin, will be tested. A reversible change in total phosphate will be examined with respect to exchange of phosphate with an NMR-invisible compartment, e.g., conversion to solid calcium phosphate within mitochondria. Non-NMR experiments of relative specific activity of total tissue phosphate with radioactivity Pi will also be conducted to investigate compartmental exchange. Additional energy sources during cardioplegia will be explored by (1) addition of inosine and pyruvate simultaneously and (2) precardioplegic administration of insulin and glucose. The metabolism of glycogen will be determined with 13C NMR spectroscopy following labelling of the glycogen with 13C-glucose. These studies should contribute to more rational procedures for cardiac surgery as well as in the use of topical NMR in clinical medicine.